Element for correcting electron-optical systems



Nov. 24, 1959 A. c. VAN DORSTEN 2,914,675

ELEMENT FOR CORRECTING ELECTRON-OPTICAL SYSTEMS Filed March 14, 1956 5Sheets-Sheet 1 1: 4s 17 16 g 4 J 4- 3 +1! ii. :0 5a. 4 0

INVENTOR ADRIANUS GJRNELIS VAN DORSTEN AGENT NOV. 1959 A. c. VAN DORSTEN2,914,575

ELEMENT FOR CORRECTING ELECTRON-OPTICAL SYSTEMS Filed March 14, 1956 5Sheets-Sheet 2 ADRIANUS CORNEUS \AN DORSTEN INVENTOR AGT NOV. 24, 1959c, VAN DQRSTEN 2,914,675

ELEMENT FOR CORRECTING ELECTRON-OPTICAL SYSTEMS Filed March 14, 1956 3Sheets-Sheet 3 EEO-t0.

INVENTOR ADRIANUS CORNEUS VAN DORSTEN AGENT it i 7 2,914,675 Jj ELEMENTFOR CORRECTING ELECTRON- OPTICAL SYSTEMS.

Adrianus Cornelis van Dorsten, Eindhoven, Netherlands,

assignor, by mesne assignments, toNorth American Philips Company, 'Inc.,New York, .N.Y., a corporation of Delaware 1 Application March 14, 1956,Serial No. 571,458 Claims priority, application Netherlands March 15,1955 a 9 Claims. (c1. 2s0 49.5

-When the field in an electron optical system is not truly radiallysymmetrical, the image produced by such a system exhibits astigmatism.As is well-known the detrimental influence 'of the asymmetry .can beeliminated by means of Correcting elements which are referred to asstigmators. The invention relates to a novel stigmator, which offerscertain advantages over the known stigmators, as will beseen from thedescription.

A fstigmator' in accordance with the invention comprises four or agreater even number of pole pieces of electrically conductivemagnetizable material which are arranged symmetrically about the samepart of an axis and each are insulated electrically from .thetwoadjacent 2,914,675 Patented Nov. 24,1959

ance with the invention provided with concave polefaces, Fig. 8 is alongitudinal sectional view of the element shown in Fig. 7 taken alongthe line VIIIVIII.

Fig. 9 illustrates the last-mentioned embodiment, and

Fig. 10 is a diagrammatic cross-sectionalview of an electron microscopein accordance with the invention.

In Figs. 1 and 2, the four convex pole pieces are designated 1, 2, 3 and4. They are made of a soft steel frequently used to manufacture magneticelectron lenses.

They are interconnected magnetically by ferromagnetic coil-cores 5, 6, 7and 8. Said cores are made of a ferromagnetic insulating material, suchas for example, one of the ferrites commercially available under thetrade 'name .ferroxcube. These materials are fully described in US.Patents 2,452,529, 2,452,530 and 2,452,531. They have the compositionMFe O in which M is a bivalent ones. 'Said pole-pieces are-providedwithcylindrical pole faces the generatrices of which are parallel to'theaxis. The stigmator must be so arranged that its axis coin-l cideswi'ththe optical axis of the system to be connected.

The pole "pieces mustbe magnetized to anequal extent so that themagnetic polarity ofeach pole-face is opposed to t hat of the twofadjacent ones; In addition, an equal electric potential must be appliedbetween each pair of adjacent pole pieces so that polefaces ofequalmagnetic polarity areat the same potential. i i f i controlling thepotential difierencs between the pole" pieces and the magnetic'fieldstrength, astigmatisms in'any direction and of difleientivalues can beeliminated without the correcting elements" or partther'eof being moed-J,

"The line along which the pole faces intersect a plane attriglitanglesto the axis may be shaped into various forms." It is of advantage forthe pole pieces to be' con vex. It has been found that the optimurnefi'ect of the stigmator in accordance with the invention is ensured byshaping the cross section of the pole faces into the form .of twoassociatedequilateral hyperbolas. I l

fA stigmator in accordance with the invention comprising four polepiecesbehaves as a combination of two cylindrical lenses in thesamespace, an electrostatic and a magnetic lens; I Thews'agittal planes orsaid cylindrical lenses are at an angle of 45 "withone another.

"In order that the invention may be readily put into effect, a number of'structuralembodiments will now be described in detail with reference to"the. accompanying drawings, in which: i i l Figl lis a part plan view,part sectional view of such an embodimennviewed in the direction of theaxis,

,Fig. 2fis, partly a side elevation of the same element, partlyasectional view thereof taken along a plane passing through the axis, i

. Figlj}3"' is apart plan view,part sectional view of a second.embodiment of astigmator in accordance with the i nv e n tion, viewedinthe direction of the axis. Fig. 4 is a cross-sectional View of theelement shown :Fig57 :is a crossf-sectionalview of a stigmato'r. inaccord-f metal such as copper, nickel, zinc, magnesium, etc. Thesematerials exhibit very low magnetic losses and have a high resistivity.Coil windings 9, 10, 11 and 12 are provided on said cores.

The pole faces are arranged symmetricallyabout the axis 13. They areprovidedwith cylindrical convex pole faces 14, 15, 16 and 17, thegeneratrices of which are parallel to the axis 13. In the operation ofthe stigmator, the current passing through the coil windings 9, 10, 11and 12 must be so directed that two adjacent coil cores are magnetizedin opposite senses. Thus, the successive pole faces 14, 15, 16 and 17are given alternate polarities: when 14 produces a north pole, 15produces a south pole, 16 a north pole again and 17 a south poleagain.Consequently the magnetic line of forcewill be symmetrical and at-rightangles to the axis 13.

In addition, an electric potential is applied between the pole pieces 1and 2. An equal potential difference is applied between the pole pieces3 and 2 and between the pole'pieces 3 and 4'. The pole pieces designatedby odd reference numerals are at the, same potential which differs fromthe common potential of the pole pieces designated by even referencenumerals. This resultsin the production of an electric field the linesof force of which coincide with the magnetic lines of force. Since thevalue of the potential must be different for any direc-. tion and valueof the astigmatism to be compensated, the potential is madecontrollable, for example between a fewhundreds of volts negative andpositive. For the same reason the magnetizing current of the coils isalso made variable so that the magneto-motive force can be controlled,for example between 100 ampere-turns negative and positive. l i

' The two fields, the electric and the magnetic field, each actas a'cylindrical lens. However, the direction of the refractive force at apredetermined point in space is not the same for the two cylindricallenses. When the curves at which the pole faces 14, 15, 16 and 17intersect the common principal plane of the two cylindrical lenses arein the form of two associated equilateral hyperboias, it can be provedmathematically that the sagittal planes of said lenses are at an angleof 45 with one another.

The astigmatism of an optical system, for example of an electron lens,produced by an insufficient degree of symmetry of the'field boundariescan be eliminated with metry of the optical system to be corrected,provided that t the lens is of suitable power. Starting from the optimumeffect of this cylindrical lens, the minimum effect is obtained byrotating it through an angle of45 only. The same eifect can be producedby using a system of two cylindrical lenses of adjustable power theazimuthal angles g of which differ by 45. This requirement is satisfiedby the insulator 34.

the correcting element in accordance with the invention which has theadvantage that it need not be rotated at all. In order to adjust theelement to the optimum effect, the power of the two component lenses ischanged in opposite senses (so that the resulting lens is rotated) untilthe remaining astigmatismexhibits a minimum value. Subsequentproportional variation of the strength of the two fields permits ofentirely eliminating the astigmatism.

When the cross-section of the pole faces is not shaped 7 into the formof a true hyperbola, the effect of the correcting element is lesscomplete, but nevertheless the use of pole pieces the shape of whichdoes not deviate excessively from the theoretically correct formprovides an improvement which is sufficient in practice. Thetheoretically correct form may, for example, be approximated to by theuse of a circular cross section.

In the embodiment shown in Figs. 1 and 2, the coilcores are arrangedtangentially. They may alternatively be arranged radially. The latterarrangement is shown in Figs. 3 and 4. The pole pieces are designated18, 19, 20 and 21. The pole faces 22, 23, 24 and 25 are shaped similarlyto those shown in the first embodiment. The coil-cores 26,27, 23 and 29in this arrangement are made of the same material as the pole pieces andare interconnected by a yoke 36 made of the same material. In order,however, to permit a voltage to be set up between the pole pieces, theyoke is provided with gaps 31, 32, 33 and 34 which are filled withelectrically insulating magnetizable material. The coil windings aredesignated 35, 36, 37 and 38. The current must pass through saidwindings in alternately opposite senses so that the magnetic flux of,for example, the core 26, starting from the pole-face 22., is closed forone half through the pole piece 19 and the insulator 31 and for theother half through the pole piece 21 and As an alternative, a stigmatorin accordance With the invention may be produced in which the coil turnsare at right angles to the axis ofthe element.

system in this embodiment comprises two inter-engaging equal parts. Saidparts eachconsist of a flat annular member (39, 40) having twodiametrically opposed projecting thickened portions (41, 42 and 43, 44respectively) the size of which in theaxial direction also exceeds thatof the remaining part of the member and the cross-section of which in aplane at right angles to the axis is shaped in the form of the twocurves of an equilateral hyperbola cut off'by a concentric circle 45.The diameter d of the circle 45 is less than the inner diameter d of themembers 39 and'4t). The members are arranged coaxially with one another,one being reversed relatively to the other, and one pair of thickenedportions is arranged symmetrically with respect to the other pair. Aferromagnetic annular member 46 made from insulating material spaces themembers 39 and 40 apart. The inner diameter d of the member 46 which isarranged coaxially with the members 39 and 40 is larger than d In thespace between the member 46 and the parts of the thickened portions 41to 44 which project beyond the surface of the members 39 and 40, amagnetizing coil 47 is arranged. Since d is less than d there is a gapbetween the thickened portion 41 and 42 and the inner wall of the member39 and likewise between the thickened portions 43 and 44 and the member40. As a result, magnetic poles are producedwhen a' current passesthrough the coil 47. The hyperbolic planes of 41 and 42 are given onemagnetic polarity and those of 43 and 44 the other magnetic polarity, sothat a magnetic cylindrical lens is produced. Between themembers39 and40 an electricpotentialcan beset up in order to make'the system into anelectrostatic cylindrical lens.

alternative embodiment of a stigmator in accordance with the invention,in which the shape of the pole faces is considerably different from thatin the embodiments so far described, is shown in Figures 7 and 8. Inthis embodiment, the pole faces 48, 49 and 50 and 51 are shaped into acircular cylinder. Fig. 9 illustrates the reason why satisfactoryresults cannevertheless be obtained with this arrangement.

When of the field produced between the hyperbolic pole faces of theabove-described embodiments the field strength in the points of a circlesituated in a plane at right angles to thejaxis and having the centre insaid axis is shown in a graph on rectangular coordinates as a functionof the angle between the radius vector and a fixed diameter, asinusoidal line is produced.

The variation of the potential along the circular crosssection of thepole faces 48 to 51 can be represented by the rectangular wave-form line52 of Fig. 9. The function represented by said line can be resolvedaccording to Fourier into a sinusoidal function and a plurality ofharmonics. For a circle having a slightly smaller radius, a curve isobtained from which the highest harmonics have disappeared and as theradius is made smaller more harmonics will disappear. In a circle havinga very small radius theyhave disappeared almost entirely, so thatsubstantially only a sinusoidal functions 53 will be left having thesame passages through zero as the initial line 52. Consequently, in theproximity of the axis the variation of the field strength isapproximately equal to that which is found throughout the entire fieldwith hyperbolic pole faces. Thus, when the radius of the cylinderconstituted by the pole faces is large compared with that of thecrosssection of the electron beam upon which the fields must act, theshape shown in Fig. 7 is useful in practice.

The stigmator shown in Figs. 7 and 8 substantially comprises twoconcentric cylinders 54 and 55 made of ferro-magnetic metal. The outercylinder tapers to- Figs. 5 and 6 "show an embodiment of thisarrangement. The magnetic wards the top and at this top. is providedwith two windows which each occupy an amount of space slightly largerthan one quarter of the periphery. In said windows tags 56 and 57 arearranged which project upwards beyond-the edge of the inner cylinder andeach occupy an amount of space slightly smaller than one quarter of theperiphery. The tops of the tags are slightly spaced away from the upperedge of the windows. Hence the two cylinders are not in contact with oneanother, although the diameters of their upper parts are equal.

The space between the cylinders, in which space a magnetizing winding 58is arranged, is closed by a ferro-magnetic electrically insulatingannular member 59. This member establishes a direct connection betweenthe cylinders for the magnetic flux which produces magnetic poles at thefaces 48 to 51.

It will be obvious that in all the embodiments de scribed the entireferro-magnetic may consist of electrically insulating material(ferroxcube), provided that the pole faces are coated with a conductivematerial so that an electric potential can be set up between said faces.

Fig. 10 is a sectional view of an electric microscope provided with astigmator in accordance with the invention.

The outer wall of said microscope is a steel tube 60 into the upper endof which an electrode system 61 is introduced which acts to produce theelectron beam which passes through the entire microscope along the axisof the tube 60. The microscope is provided with three magnetic lenses: acondenser 62, an objective 63 and a projection lens 64. The design andthe operation of said members are assumed to be known. In the objective63, an object 65 is arranged an image of which is produced by theelectron beam upon the glass window 66 which is coated with a layer 67of fluorescent material.

According to the invention, between the objective and the projectionlens provision is made of the correcting element 68 which serves toeliminate the astigmatism of the objective in an optimum manner.

secured to this element and passed out through the wall 60. In addition,provision is made of electric conductors '70 which act to supply currentto the magnetizing coils. By adjusting the potential and the current thepower of the two cylindrical lenses produced by the element can beadjusted. This electric adjustment replaces the known adjustment bymeans of displacement in the tube of metal correcting members. Thismeans a considerable simplification.

Since the electrostatic cylindrical lens exhibits no sphericalaberration and the magnetic lens shows only little spherical aberration,the electron beam in the stigmator may have a large cross-section, whilea comparatively large distance between the correcting element and thelens with which it cooperates is not inconvenient per se. For the samereason, an increase in the axial size of the element does not adverselyafiect the image quality. A comparatively large size in the direction ofthe axis is even of advantage since it reduces the influence ofinconvenient marginal fields.

1 However, it is desirable for other reasons that the distance betweenthe stigmator and the objective be small compared with that between thestigmator and the production lens, for the smaller the latter distanceis at a predetermined distance between the objective and the productionlens, the stronger the stigmator must be magnetized in order to enableit to eliminate the astigmatism, but the stronger its influence becomesupon the magnification produced by means of the system to be corrected(obviously in one direction only).

By increasing the spacing between the stigmator and the projection lens,the distortion of the image is reduced. For this reason the stigmator isembedded in the ferromagnetic of the objective in the microscope shownin Fig. 10.

What is claimed is:

1. An electron optical system for an electron microscope having a givenoptical axis comprising electron beam producing means, an electron lenssystem for focussing the electron beam which introduces astigmatism intothe focussedbeam along said axis, a plurality of pairs of spaced polemembers each of magnetizable material disposed symmetrically about saidoptical axis for compensating for the astigmatism introduced into thebeam by the electron lens system, each of said pole members having acylindrical pole face the generatrix of which is parallel to said axis,a member connecting each of said pole members to one another andconsisting of an electrically-insulating ferromagnetic material having acomposition MFe O in which M is a bivalent metal, means to alternatelypolarize each of said pole members in an opposite sense, means to applyequal electrical potential difierences between adjacent pole members,and means for varying the potential difierence between and the fieldstrength of successive pairs of pole members.

2. An electron optical system as claimed in claim 1 in which theelectron lens system is a magnetic lens system.

3. An electron-optical system as claimed in claim 1 in which thecylindrical pole face is convex.

4. An electron-optical system as claimed in claim 3 in which a sectionof each pair of pole faces forms two associated hyperbolas in a plane atright angles to the common axis.

5. An electron-optical system as claimed in claim 3 in which thepole-face is a circular cylinder.

6. An electron-optical system as claimed in claim 1 in which the polemember polarizing means is a coil disposed on said pole and has a givenaxis which is tangent to a circle concentric with the common axis andpasses through said pole member.

7. An electron-optical system as claimed in claim 1 in which each pairof pole members together comprises two coaxial electrically conductiveannular members secured together by a yoke of electrically insulatingferromagnetic material having a composition MFe O in which M is abivalent metal.

8. An electron-optical system as claimed in claim 7 in which the polemembers are polarized by a coil disposed between the annular members.

I 9. An electron-optical system as claimed in claim 7 in which the polemembers are polarized by a coil surrounding the annular members.

References Cited in the file of this patent UNITED STATES PATENTS2,200,039 Nicoll May 7, 1940 2,486,856 Liebmann Nov. 1, 1949 2,520,813Rudenberg Aug. 29, 1950 2,580,675 Grivet et al Jan. 1, 1952 2,586,559Page Feb. 19, 1952

1. AN ELECTRON OPTICAL SYSTEM FOR AN ELECTRON MICROSCOPE HAVING A GIVENOPTICAL AXIS COMPRISING ELECTRON BEAM PRODUCING MEANS, AN ELECTRON LENSSYSTEM FOR FOCUSSING THE ELECTION WHICH INTRODUCES ASTIGMATISM INTO THEFOCUSSED BEAM ALONG SAID AXIS, A PLURALITY OF PAIRS OF SPACED POLEMEMBERS EACH OF MAGNETIZABLE MATERIAL DISPOSED SYMMETRICALLY ABOUT SAIDOPTICAL AXIS FOR COMPENSATING FOR THE ASTIGMATISM INTRODUCED INTO THEBEAN BY THE ELECTRON LENS SYSTEM, EACH OF SAID POLE MEMBERS HAVING ACYLINDRICAL POLE FACE THE GENERATRIX OF WHICH IS PARALLEL TO SAID AXIS,A MEMBER CONNECTING EACH OF SAID POLE MEMBERS TO ONE ANOTHER ANDCONSISTING OF AN ELECTRICALLY-INSULATING FERROMAGNETIC MATERIAL HAVING ACOMPOSITION MFE2O4 IN WHICH M IS A BIVALENT METAL, MEANS TO ALTERNATELYPOLARIZE EACH OF SAID POLE MEMBERS IN AN OPPOSITE SENSE, MEANS TO APPLYEQUAL ELECTRICAL POTENTIAL DIFFERNECES BETWEEN ADJACENT POLE MEMBERS,AND MEANS FOR VARYING THE POTENTIAL DIFFERENCES BETWEEN AND THE FIELDSTRENGTH OF SUCCESIVE PAIRS OF POLE MEMBERS.